Many English words have more than one potential pronunciation. Accent differences play a part in this - for example the /ʌ / sound used in Southern British English becomes /ʊ/ in Northern BE, so that eg bus may be pronounced /bʌs / or /bʊs/.
However, there may also be a difference between the citation form of a word - ie how it would be pronounced slowly and in isolation, and its usual pronunciation in spoken discourse, where it will show features of connected speech such as elision and vowel weakening.
For example - take the word library. Said individually, it would probably be pronounced /laɪbrəri/, but in connected speech might become /laɪbri/.
Most grammatical words (prepositions, conjunctions, auxiliary verbs, pronouns, quantifiers and articles) will have both a citation form and a second form (weak form) used in connected speech when the word is in unstressed position. But as the example of library shows, lexical items may also be affected.
A few other examples of lexical items with a citation form distinct from their normal connected speech pronunciation include :
probably : citation form = /prɒbəbli:/; connected speech = / prɒbli:/
foliage : citation form = /fəʊli:jɪʤ/; connected speech = /fəʊlɪʤ/
supposed : citation form = /səpəʊzd/; connected speech = /spəʊzd/
interesting : citation form = /ɪntərestɪŋ/; connected speech = /ɪntrestɪŋ/